1. Field of the Invention
This invention relates to a smelting reduction process for producing molten iron from an iron oxide material, by feeding an iron oxide material such as iron ore and prereduced iron to a smelting reduction furnace along with solid carbonaceous material, a fluxing agent and an oxygen-containing gas for smelting reduction, and more particularly to a smelting reduction process which can reduce slag production to a marked degree to lessen the operational load of smelting reduction furnace and permits production of high quality molten iron of low phosphorus and sulfur contents.
2. Description of the Prior Art
As iron-making technology other than blast furnace processes, the so-called smelting reduction process has come into the limelight, urging developments of various processes in this connection, including COREX process, XR process, SC process, COIN process etc.
Generally in iron-making by the smelting reduction process, a CO-dominant hot gas which is generated in a smelting reduction furnace is fed to a prereduction furnace as a heat source and at the same time as a reductant to produce prereduced iron with low or, if necessary, high prereduction degree in the prereduction furnace, while adding carbonaceous material and blowing oxygen-containing gas into the smelting reduction furnace to supply heat thereto and melting and finally reducing the prereduced iron which is being supplied from the prereduction furnace.
The prior art concerning such smelting reduction process includes: (1) A process of charging prereduced iron from a prereduction furnace to a smelting reduction furnace, blowing coal and oxygen into an iron bath held in the smelting reduction furnace to smelt and reduce the iron ore while burning part of the generated gas on the bath (post-combustion), and reforming the gas drawn off the smelting reduction furnace before introducing same into the prereduction furnace (Japanese Laid-Open Patent Application No. 59-222508); (2) A process of preheating iron ore, prereducing the preheated iron ore in a prereduction furnace, blowing the prereduced iron ore into a smelting reduction furnace along with a carbonaceous material, oxygen and a fluxing agent thereby secondarily burning part of the fuel and heating the iron bath produced in the smelting reduction furnace, while simultaneously cooling and decarbonating the produced reducing gas to adjust its oxidation degree before introducing same into the prereduction furnace for control of the prereduction rate (Japanese Laid-Open Patent Application No. 60-145307); and (3) A process which is slightly different in principles from the just-mentioned two prior art processes and which employs a converter (an iron bath type smelting reduction furnace) without a prereduction furnace, obtaining pig iron by adding agglomerates of iron ore and a carbon source and quicklime to the converter together with oxygen blowing (Japanese Patent Publication No. 57-40883).
The present inventors disclosed "Method and Apparatus for Blowing Solid Fuel into Electric Furnace or Converter" in our prior application, Japanese Laid-Open Patent Application No. 62-267407.
(a) The invention of this prior application concerns an improvement in power consumption in electric furnaces and low-cost heat compensation in converters, namely, it is restricted to an auxiliary measure of replacing part of the processing heat source by a solid fuel and applicable in a different range from that of the present invention related to the smelting reduction process.
(b) Although the process of the above mentioned prior application is directed specifically to electric furnaces and converters, the present invention is not restricted to the furnaces of these types.
(c) The product of the process in the prior application is molten steel, while the product of the present invention is molten iron, especially molten iron with a carbon concentration higher than 2%.
(d) The present invention combines desulfurization in a precombustor to solve the problems resulting from the use of solid carbonaceous material, permitting to produce molten iron of high quality. This is because the problem of S is important in the present invention where a carbonaceous material as mentioned in (a) above is used in a wide range (i.e., in a larger amount).
Further, we have filed a patent application (Japanese Laid-Open Patent Application No. 63-28818 for "Method and Appratus for Blowing Fuel into Electric Furnace & Converters". In this prior application, the fuel is burned completely outside a furnace, and the exhaust gas is injected against the raw material in the electric furnace or converter, without the concept of primary and secondary combustins as in the present invention.
We have also filed a patent application (Japanese Laid-Open Patent Application No. 63-72814) for "Electric Furnace Steel Making Process". As the title implies, this application is directed to a process by electric furnace into which combustion exhaust gas is blown similarly to the just-mentioned prior application, likewise without the concept of primary and secondary combustion as in the present invention.
In the smelting reduction iron making processes, particularly in the smelt reduction furnace operations as mentioned in (1) and (2) above, it is necessary to add a large amount of fluxing agents such as limestones and burnt lime for removal of the sulfur and phosphorus brought in mainly by the solid carbonaceous material and the iron ore, for suppression of slag foaming and for protection of the refractory material of the furnace. Namely, these fluxing agents function as desulfurizer, dephosphorizer and a coolant which prevents slag foaming, and from the standpoint of promoting the desulfurizing and dephosphorizing reactions, the fluxing agent has an important role as a basicity adjusting agent.
3 to 13% of gangue content in the iron oxide material like iron ore and 5 to 15% of ash content in the solid carbonaceous material are mostly constituted by silica, and acidic component, in contrast to an extremely small basic content. Therefore, the slag has a trend toward the acidic side, and, in order to form a basic slag by counteracting this trend, it is necessary to charge a large amount of basic fluxing agent. This naturally increases the amount of slag and the load of the smelting reduction furnace operation to a considerable degree.
FIG. 4 shows the secondary combustion rate in the iron bath type smelting reduction furnace in relation with coal and lime consumptions and the amount of slag, giving the figures in those cases where the basicity was adjusted in such a manner as to hold the S concentration in molten iron at 0.15% to lessen the load in the subsequent desulfurization and refining stages. As clear therefrom, increases of secondary combustion ratio causes drop in coal feed and accordingly by decreases in S and ash which are supplied mainly by coal and drops in lime feed and amount of slag. However, as indicated by * in the same figure, the amount of slag reaches 200 kg/T even under a relatively low load condition of the smelting reduction furnace operation where metallization of the prereduced iron is 70% and post-combustion ratio is 25%, producing an extremely large amount of slag as compared with ordinary converter operations.
Further, in a process using a system having a smelting reduction furnace directly coupled with a prereduction furnace (hereinafter referred to as "once-through system"), the amount of slag at a balancing point of operation becomes, for example, greater than 300 kg/T as seen in FIG. 14, giving rise to problems to be solved, namely, problems such as an increase in consumption of the refractory material of the smelting reduction furnace, a drop in the iron yield and an increase of the volumetric furnace capacity for securing the freeboard.
In most of the above-mentioned smelting reduction furnaces, a large amount of pulverized coal is injected into the molten iron bath. Therefore, the bottom blowing equipments which serve for this purpose, including the bottom blowing tuyeres, pipings and refractory materials around the tuyeres, considerably increase the burden of maintenance.
Further, the quality of the ultimate iron product is a serious problem exists in the quality of the iron product of the prior art processes in which a solid carbonaceous material is directly introduced into a smelting reduction furnace. Namely, the P and S levels in the molten iron which is produced in the smelting reduction furnace are determined by the P and S contents in the feed to the furnace minus the P and S contents which are discharged out of the molten iron along with the slag. However, it is difficult to remove both P and S efficiently by way of the slag. In order to have high dephosphorization capacity, it is necessary for the slag to be high in basicity and ozygen potential. On the other hand, in order to have high desulfurization capacity, it is necessary for the slag to be high in basicity and low in oxygen potential. High basicity of slag is effective for both dephosphorization and desulfurization but it has limitations in view of the following problems: the slag will have a higher melting point with lower fluidity which makes its discharge from the furnacce difficult; the basic fluxing agent such as limestone will have to be used in a large amount; and as a result the amount of slag will be increased to lower the operating efficiency. Thus, there are limitations on the degree to which the slag can be improved with respect to both dephosphorization and desulfurization. The slag is superior in dephosphorization but inferior in desulfurization or vice versa depending upon its oxygen potential. For example, in case of a low shaft type smelting reduction furnace packed with a carbonaceous material or a conventional blast furnace process, it is possible to obtain low oxygen potential (e.g., about 0.5% of FeO concentration in slag). Therefore, the S level in molten iron shows a relativeluy low value of 0.04%, but the P level reaches an extremely high value of 0.08-0.12%, increasing the dephosphorization load in the refining process in a converter or the like. On the other hand, in case of a converter type iron bath smelting reduction furnace, for example, the oxygen potential of slag becomes relatively high, so that it is possible to hold the P level as low as about 0.02%, but the S level reaches a value greater than 0.1% and in some cases reaches about 0.3%. Therefore, a large load is imposed for desulfurization of the molten iron, which is a negative factor to the production cost. Thus, the smelting reduction process which directly uses a solid carbonaceous material invariably has a serious problem with regard to either the P level or S level of the molten iron.